Old and single, share in vindication (11)
I believe the answer is:
exoneration
'vindication' is the definition.
(I know that vindication can be written as exoneration)
'old and single share' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'and' says to put letters next to each other.
'single' becomes 'one' (a single thing - only one).
'share' becomes 'ration' (a ration is a share of a limited resource).
'ex'+'one'+'ration'='EXONERATION'
'in' acts as a link.
(Other definitions for exoneration that I've seen before include "Act of absolving from blame" , "get acquittal" .)